This invention relates to an optical memory device and more particularly to an optical disc for recording and reproduction of data by laser light.
Optical discs are being developed as large-capacity memory means. Optical discs of the add-on type (that is, the draw-type discs having TeO.sub.x and the like as the medium) and the rewritable type (that is, magnetooptical discs with rare earth-transition metallic alloy as the medium) have grooves for recording and track number sections where track position information is recorded by providing small holes, or "pits" to the substrate as described, for example, in U.S. patent application Ser. No. 767,537 now abandoned and assigned to the present assignee.
FIG. 2 is a plan view of a small portion of an optical memory device, showing one of its grooves 1 and a few of such pits 2. An output signal representing a track number recorded in the form of pits is illustrated in FIG. 3 and such a track number signal is generally amplified by an AC amplifier, not a DC amplifier. If this track number signal is sliced at the center as shown in FIG. 4(a), or the zero-level, to convert it into a digital signal, a waveform as shown in FIG. 4(b) may be obtained. By a signal processing procedure of this type, however, there are situations in which the result looks as shown in FIG. 5. This may happen, for example, when the upward amplitude L.sub.1 of the track number signal and its downward amplitude L.sub.2 are unequal as shown in FIG. 3, because the DC component of the track number signal is different between the pit section and the groove section, and the transient response of the AC amplifier distorts the envelope of the signal at the beginning as shown by the numeral 4 in FIG. 5. If such a distorted signal is sliced at the zero-level of the signal as a whole as explained above, the signal is not sliced at the actual zero-level in the area where the envelope is distorted, and the digital signal thus obtained will be an incorrect one.
An optical device having such an AC amplifier and using such an optical disk with grooves and pits has been disclosed, for example, in Japanese Patent Publication Tokkai 60-127536 filed Dec. 14, 1983 and published Jul. 8, 1985, which publication will be herein incorporated by reference.